How is the percentage of NOD calculated?

The correct formula for calculating the percentage of NOD (Non-Organic Demand) in relation to BOD (Biochemical Oxygen Demand) is based on the proportion of each parameter. The formula %NOD = (NOD / BOD) x 100 accurately expresses this relationship by comparing the amount of NOD to the total BOD, allowing you to express NOD as a percentage of BOD.

Understanding this relationship is crucial, especially in wastewater treatment, because BOD measures the total oxygen demand from both organic and non-organic materials in the water. By using this formula, you can quantify how much of the total oxygen demand is attributed to non-organic substances, which is essential for assessing treatment processes and overall water quality.

The calculation included in other choices, such as subtracting NOD from BOD or adding NOD to BOD, does not appropriately reflect the relationship between these two metrics. Instead of giving a percentage that accurately shows the contribution of NOD to BOD, those options misrepresent how to express NOD as a fraction of the total oxygen demand. Therefore, understanding the correct relationship embodies the essence of the calculation and its significance in wastewater analysis.